A microlens for an imaging device such as a charge coupled device (CCD) can produce a high-definition imaging device mainly by forming a fine pattern and is produced using a method for forming a pattern by a photoresist. In this method, a microlens is produced by a method including: applying a resist composition containing a polymer resin and a photosensitizer on a substrate to make a film of the resist composition; patterning the resultant film by a photolithography method; and developing the film to form one pattern. For the formed microlens pattern, there are required high transparency and heat resistance because the microlens is exposed to a high temperature condition in a soldering process. Further, for the microlens pattern, it is required to have a desired curvature radius and a high refractive index from the viewpoint of the resolution.
In an optical device such as a liquid crystal display device, an LED display device, and a solid-state imaging device, there is used a planarization film for planarizing a substrate surface and a solid-state imaging device surface of a color filter or the like. For the planarization film, planarizing properties, adhesion to a substrate surface and an element surface, and light transmittance (transparency) are required, and further, for preventing the exposure of a substrate surface and an element surface to heat during a production process of an optical device, a function as a protecting film having heat resistance is also required.
As a resist composition for forming the microlens as described above, there is disclosed a resist composition using a polymer having, as a repeating unit, hydroxystyrene, hydroxyvinylnaphthalene, or hydroxyanthracene in which hydrogen atoms of hydroxy groups are partially substituted with an acid unstable group (for example, Patent Document 1). However, a pattern formed from such a resist composition does not have heat resistance sufficient for an imaging device application.
On the other hand, as a high refractive index resin for optical materials having high heat resistance, there is disclosed a high refractive index resin for optical materials containing a copolymer produced by copolymerizing a single type or a plurality of types of vinyl monomer radical-polymerizable with 4-vinylbiphenyl (for example, Patent Document 2). However, though this high refractive index resin for optical materials has high heat resistance and a high refractive index, the resin does not have a resolution sufficient for an imaging device application.